They won smashing victories last week in Virginia and other states. With voters giving the Trump presidency and the Republican-led Congress dismal grades, and the Democratic grass roots re-energized, hope is widespread for a takeover of the House of Representatives and a strong run in the Senate in the 2018 midterm elections.

But for all the optimism, the elections in Virginia last week vividly reflected why the reality might be a good deal harsher. While Democrats won the governorship by nearly nine percentage points and won a similar margin in total votes in legislative races, it appears likely, unless recounts reverse seats, that they will fall just short of taking control of the state’s heavily gerrymandered House of Delegates.

And around the country, gerrymandering, refined to a high art, and increasingly restrictive voting laws have left many experts wary of assuming that the intensity of Democratic voters will translate into equally robust electoral gains.

For some, the lesson of Virginia is that grass-roots organizing and voters eager to turn out can pull off big wins in unlikely places. But for others, the gap between votes and legislative seats is a cautionary reminder that Democrats face daunting structural obstacles in turning around Republican majorities in Congress and in state legislatures.

“If Democrats win 52, 53, 54 percent of the national House vote, we’re likely to see Republicans hold onto control,” Nicholas Stephanopoulos, a University of Chicago law professor and an expert on gerrymanders, said in an interview. “Unless there’s a true wave, I think Democrats will be disappointed in 2018.”

Republicans scoff at the notion that political gerrymandering, a bipartisan sport nearly as old as the Republic, presents a meaningful barrier to Democrats’ chances.

“What you’ve got there is a classic fig leaf by the Democrats to explain their ineptitude and lack of focus and ultimate lack of ability to convince the voters they have a plan to move forward,” said Matt Walter, president of the Republican State Leadership Committee, which focuses on statehouse races. The committee’s success in winning state legislative seats around the country in 2010 helped Republicans draw favorable congressional districts in state after state.

The Democratic Party’s political challenges go well beyond the structure of American elections: The party’s bench of candidates is depleted after several electoral wipeouts. Divisions between the Democratic establishment and the party’s activist base threaten to yield a rocky primary season. In large swaths of the country, Mr. Trump remains a popular figure, likely limiting the inroads Democrats can make into rural areas where their appeal has faded in recent years.

What’s more, the electoral landscape is tilted against them for reasons besides gerrymandering. Liberal voters and racial minorities tend to be clustered in major cities and their suburbs, concentrating the Democratic base in a smaller number of congressional districts, even when the districts are drawn in an evenhanded way. And in the Senate, the list of seats Democrats are defending in 2018 is overwhelmingly anchored in rural red states like West Virginia and Montana — largely because of political coincidence.

But few experts doubt that gerrymanders and restrictive voting laws will have a substantial impact in 2018, and strategists on both sides agree that Democrats have only a narrow path to capture a majority in the House.

Eric H. Holder Jr., the former attorney general who is leading a Democratic effort to challenge Republican-drawn maps, said he believed Democrats would be poised for monumental gains “if the lines were drawn in a fair way.” As it is, he said, Democrats might need not merely a friendly tide, but a political rip current to win power in Washington.

“It’s almost going to take a historic wave to overcome the gerrymandered map in a handful of states,” Mr. Holder said. “When you have a massive Democratic wave in Virginia and the question of who has the majority in the House of Delegates is still up in the air, I think that shows how daunting the task is going to be.”

Still, Mr. Holder added firmly, “The House of Representatives is certainly in play.”

An analysis by Mr. Holder’s group, the National Democratic Redistricting Committee, found that Democratic candidates for the House of Delegates in Virginia received about 224,000 more votes than Republicans, out of about 2.4 million cast. Several races are still undecided and headed for recounts, but for now Republicans have a 51-49 advantage.

Mr. Walter of the Republican legislative committee said the Holder analysis “makes no sense” because it ignores how geographically concentrated Democratic voters are.

“We don’t have a proportional representative system,” he said. “It’s based on the amount of people and population drawn over the state.”

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An election official checking a voter’s identification in Austin, Tex. Seventeen states now require voters to show photo IDs at the ballot box.CreditEric Gay/Associated Press

Striving to expand the political playing field, Democrats have announced a target list of 80 House seats, including clearly conservative-leaning districts in suburban and rural areas of Ohio and Wisconsin, which have congressional maps tilted most conspicuously toward Republicans. They must take over two dozen Republican districts to win control of the House.

But in a sign of their fundamental advantages, Republicans have a far shorter list of races that concern them, and multiple party strategists said they believed there were only 35 to 40 Republican lawmakers in seats that Democrats could seize. In some of the most populous states in the country — including Michigan, Pennsylvania and Texas — Republicans believe they can lose no more than two or three seats, and perhaps fewer.

Former Representative Thomas M. Reynolds, Republican of New York, who led the National Republican Congressional Committee in 2006, said the congressional map was far better for Republicans now than it was a dozen years ago, the last time a Democratic wave election tipped control of the House.

“Redistricting has strengthened both state chambers and the Congress, and that is much more significant in 2017 than it was in 2006,” Mr. Reynolds said. “Look how many seats are in play. It’s not that many.”

In addition to gerrymandering, Mr. Holder and other Democrats have warned insistently about the impact of state-level voting procedures that threaten to disadvantage lower-income and minority voters. The Democratic Congressional Campaign Committee has begun to track states and localities where Republicans have total control of election administration, and Democratic aides say they plan to assemble lawyers around the country to monitor election machinery for “red flags” of partisan interference.

Yet the party’s ability to roll back the most significant voting restrictions may be limited. While federal courts have invalidated or reined in some of the most stringent restrictions — in North Carolina, Texas and Wisconsin — there are 17 states that now require voters to show photo identification at the ballot box. The types of acceptable identification vary, but Democratic-leaning minorities and young people are least likely to possess them.

The impact of those restrictions is difficult to measure, but experts say they do deter would-be voters from casting ballots. In Texas, where Democrats are targeting four House seats, Stephen Ansolabehere, a political scientist at Harvard University, analyzed the state’s voter rolls for plaintiffs in a federal lawsuit and concluded that 608,000 of the 13.5 million registrants lacked any of the seven IDs accepted at polling places. Black voters were two to three times as likely as whites to have none of the identifications; Hispanics one and a half to two times as likely.

“It affects all poor people, basically,” Professor Ansolabehere said in an interview.

If Virginia — which has a voter identification law that is less restrictive than many in place — appears to encapsulate many of the Democrats’ disadvantages, more optimistic veterans of the campaign say its positive lessons are more useful.

Strategists and activists argue that some campaign tactics the party employed there can be scaled up for the midterm elections, increasing turnout in exactly the communities Democrats must engage. Liberal activist groups deployed large numbers of volunteers to knock on doors, recruit candidates and register voters and raised large sums of money online. In the end, the party made gains in the Legislature far beyond its expectations — even as it fell short of a full takeover.

In the case of Wendy Gooditis, who won a long-shot victory over a well-liked Republican incumbent in the Washington exurbs, volunteers from chapters of Swing Left and Indivisible, national activist groups, showed up every weekend.

“We knocked on every single door in this district,” said Emma Brown, Ms. Gooditis’s campaign manager. “We’d hear over and over: ‘I’ve always voted for Republicans. I just can’t do it this year.’”

Ms. Brown, 24, has already moved on to the next campaign. She is the campaign manager for a Democrat challenging Representative Barbara Comstock, a vulnerable Republican in Northern Virginia, who holds a seat whose boundaries are drawn to favor her party.

Whether some of these roadblocks for Democrats disappear or grow even larger depends on the courts and the Justice Department, which appears to be abandoning the emphasis on voting-rights expansion, the rule under President Obama.

Should the Supreme Court strike down egregious gerrymandering early next year, Mr. Stephanopoulos said there was a faint chance that some states with the most partisan maps could be forced to redraw them before the 2018 election. In one of those states, Pennsylvania, voting-rights advocates last week secured fast-track consideration of a state lawsuit that challenges the state’s House of Representatives map.

The legal outlook for some other restrictions on the ballot is murkier. Under Attorney General Jeff Sessions, the Justice Department dropped its Obama-era claim that the voter identification law in Texas, one of the nation’s most stringent, had a discriminatory purpose — even though both a district court and an appeals court had ruled that it did. Voting-rights advocates call that a clear signal to states that the federal government is uninterested in challenging restrictions on voting.

Even when Mr. Obama was in power, the Supreme Court’s decision to strike down the heart of the 1965 Voting Rights Act ended its veto power over changes in election procedures in a range of mostly Southern states. Left unchecked, local officials in many of those states engaged in a range of tactics last year that likely would have been barred in earlier years; whether the Justice Department’s recent turnabout will spur more such actions remains to be seen.

The Supreme Court is scheduled to hear an important case on whether Ohio’s government can purge its rolls of voters simply because they have failed to vote. A ruling that they can would be likely to encourage some states to mount aggressive purges of their rolls, a process conservative advocacy groups already are pressing nationwide. Experts say such purges disproportionately affect Democratic registrants.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Voting Barriers Could Undercut Democratic Wave. Order Reprints | Today’s Paper | Subscribe